There are significantly improved required peak rates of a Long Term Evolution-Advanced, LTE-A, system up to 1 Gbps in the downlink and 500 Mbps in the uplink as compared with an LTE system. The LTE-A system is also required to be well compatible with the LTE system. Carrier Aggregation, CA, has been introduced to the LTE-A system in view of the required improved peak rates, compatibility with the LTE system and full use of spectrum resources.
With carrier aggregation, a user equipment can operate concurrently over a plurality of cells, where these cells can be consecutive or inconsecutive in frequency, and bandwidths of the respective cells may be the same or different. There is a limited bandwidth up to 20 MHz of each cell for compatibility with the LTE system. The number of cells that can be aggregated for the user equipment is typically up to 5 at present.
In the carrier aggregation system, all of cells configured by a eNB for the user equipment can be referred to as serving cells, but all the functions of the different cells may not be the same, so the serving cells are further categorized in the LTE-A system as follows:
A Primary Cell, PCell, where only one of the plurality of cells aggregated for the user equipment is defined as a Pcell, which is selected by the eNB and configured to the user equipment by Radio Resource Control, RRC, signaling. A Physical Uplink Control Channel, PUCCH, is configured only over the PCell; and
A Secondary Cell, SCell, where all the other cells than the PCell aggregated for the user equipment are SCells.
With the concept of carrier aggregation in the Release 10, R10 /Release 11, R11, only the cells served by the same eNB can be allowed to be aggregated for a User Equipment, UE, that is, intra-eNB (intra-eNB) aggregation.
Both the LTE and LTE-A systems are scheduling-based systems, where the eNB allocates time and frequency resources to the UE for data transmission, and the user equipment receives downlink data or transmits uplink data according to a scheduling command of the eNB.
Uplink data transmission is scheduled by the eNB, where a scheduler of the eNB notifies the user equipment of an uplink resource by an uplink, UL, grant after determining the allocation of the uplink resource. The scheduler of the eNB allocates the uplink resource in accordance with the amount of uplink data to be transmitted by the UE, i.e., a buffer state of the UE. The buffer is at the UE side, and the UE needs to make a Buffer State Report, BSR, to the eNB so that the eNB has knowledge of the state.
As described above, carrier aggregation prior to the Release 11 , R11, refers to aggregation of cells served by the same eNB, i.e., intra-eNB aggregation. The Release 12, R12, may have inter-eNB aggregation introduced thereto. Inter-eNB aggregation has the following two modes:
In a mode 1, the same RB of the same UE is transmitted by different eNBs.
In a mode 2, different RBs of the same UE are transmitted by different eNBs.
For the UE of the R11 and earlier releases, a BSR, is made based upon the size of a buffer of each logic channel group reported by the UE, but if inter-eNB aggregation is introduced to the R12, then two eNBs can perform uplink scheduling respectively, and apparently the existing BSR scheme is not applicable to the scenario of inter-eNB aggregation.
In summary, there has been absent so far a solution to making a buffer state report in the scenario of inter-eNB aggregation.